The publications and other materials used herein to illuminate the background of the invention, and in particular, cases to provide additional details respecting the practice, are incorporated by reference.
Cardiac troponins I and T (cTnI and cTnT are sensitive and reliable markers of myocardial damage and their use has been recommended by scientific committees for the diagnosis of acute coronary syndromes (ACS)(1, 2). The complex molecular nature of cTnI complicates the measurements by immunoassays giving rise to large discrepancies in cTnI concentrations as measured with differently configured commercial and research assays (3, 4). This is conceivably a compound effect of the lack of an international standard and the divergent recognition of the numerous cTnI forms by the antibodies employed in the assays (5, 6). Since cTnI is a component of the troponin complex responsible for the regulation of muscle contraction, it interacts with other components of the complex, cTnT and troponin C (TnC) with a strong interaction with TnC in the presence of intracellular calcium (7). It has been shown that the main part (about 90%) of cTnI in the blood of patients with acute myocardial infarction (AMI) is in the form of the binary cTnI-TnC complex and only a small part is in the free form (8, 9). The antibodies used in immunoassays should ideally recognise both free and complexed cTnI in an equimolar manner (10) but more specifically the unrestricted recognition of the complex is of vital importance. Moreover, the use of antibodies against epitopes in the central part of cTnI has been generally recommended since the N- and C-terminal ends are susceptible to proteolytic degradation both in vivo and after sampling (10-12). The cTnI molecule contains two serine residues in positions 23 and 24 that can be phosphorylated in tissue, and it has been reported that around 50% of the cTnI in the blood of AMI patients is in the phosphorylated form (13). The structure and conformation changes following phosphorylation can significantly affect the binding of some antibodies to cTnI (10, 14). Furthermore, the two cysteine residues in positions 80 and 97 can form a disulfide bond allowing cTnI to be found in oxidized and reduced forms (15) again affecting the antigen recognition of different commercially available assays (6). Since the cTnI molecule has a high positive charge (pI 9.87) it will attract negatively charged molecules such as heparin, which in turn can interfere with the antibody-antigen interaction (10). Heparinised plasma samples are widely used in clinical laboratories and thus discrepant results compared to serum will occur if the antibodies are affected by heparin (16). Also EDTA used as anticoagulant can cause discrepancy, especially in assays utilising antibodies with different recognition of free and complexed cTnI (17), since EDTA will partly unfold the calcium-dependent troponin complex. All of the characteristics of cTnI mentioned above should be thoroughly considered in the design and development of cTnI immunoassays.
Analytical interference due to a number of more non-specific causes is a common problem that affects almost all immunoassays to some extent, resulting in false-positive or false-negative results (18). The most frequently described interfering mechanisms are those caused by heterophilic antibodies, human-anti-mouse antibodies, rheumatoid factors and complement [reviewed in (19, 20)]. Introduction of blocking agents (e.g. mouse serum or IgG) is often useful in eliminating this type of interference (21, 22). Especially false-positive results for cTnI have been frequently reported, while the number of false-negative results reported is much lower. The false-positive results have been claimed to be due to the presence of heterophilic antibodies (23), rheumatoid factor (24, 25) or an unknown macromolecular complex (26). Unrepeatable false increases have also been attributable to the presence of fibrin after incomplete separation of serum (27). More exceptionally they may result from fluid therapy given to patients (28). One report of a false-negative cTnI result was attributed to circulating cTnI autoantibodies (29) and also bilirubin and hemoglobin have been reported to give negative interference in certain cTnI assays (30).